The classical approach to manufacturing nitrous oxide (N2O) is via ammonium nitrate decomposition. Nitrous oxide is a by-product in nitric acid production process. Nitrous oxide is also produced in large volumes as a by-product in the synthesis of adipic acid; one of the two reactants used in nylon manufacture.
Adipic acid is a dicarboxylic acid manufactured by a two-stage process. The first stage of manufacturing usually involves the oxidation of cyclohexane to form a mixture of cyclohexanone and cyclohexanol. The second stage involves oxidizing the mixture with nitric acid to produce adipic acid. Nitrous oxide is generated as a by-product of the nitric acid oxidation stage.
On the basis of experiments, the overall reaction stoichiometry for N2O production in the preparation of adipic acid was estimated at approximately 0.3 kg of N2O per kilogram of the product.
With such large volume, the by-product from the synthesis of adipic acid can become a major commercial source for N2O. However, recovering N2O from the off gas stream from the synthesis of adipic acid requires the removal of various impurities; such as, higher oxides of nitrogen, carbon dioxide, carbon monoxide, nitrogen, oxygen, argon, moisture, and organic impurities. Currently much of the off gas stream is decomposed before release for environmental protection.
Work has been done either to recover and purify nitrous oxide for different purposes or to deplete nitrous oxide from industrial off gas streams for environmental protection.
A conventional nitrous oxide purification system and process is described in EP 0636576 A1. More specifically, the processes starts with compressing a nitrous oxide feed gas that contains nitrogen, oxygen and water in a compressor. A portion of nitrous oxide, free of water, is conveyed to a heat exchanger where it is condensed to a liquid. The liquid gas is directed to the top of a rectification column for further condensation and returned to the bottom of the rectification column. A second mixed gas-liquid nitrous oxide portion is directed from the heat exchanger to the bottom of the rectifying column as a heating source to vaporize the liquid. The same nitrous oxide is then directed out of the bottom of the column and introduced to the middle of the column. The final product is taken from the bottom of the column having the requisite purity.
One of the disadvantages associated with such a system is that it is complex, as it includes three heat exchangers and two rectification sections. In addition, the compressor employed is potentially a secondary source of contamination due to the lubricating oils needed to operate it.
An invention relates to processes for the separation of nitrous oxide from feed streams containing mixtures thereof with oxygen and nitrogen is described in U.S. Pat. No. 6,080,226. In the process, the feed stream comprising nitrous oxide is purified by a pressure swing adsorption (PSA) process employing a co-purge with an oxygen-lean stream to produce a high purity nitrous oxide stream. Nitrous oxide is selectively adsorbed, thus the first effluent stream contains N2 and O2. The high purity nitrous oxide stream can be incorporated in a complex for the production of adipic acid to recover nitrous oxide from a dilute waste stream and pass the recovered nitrous oxide to a process for the production of phenol from an aromatic hydrocarbon. Unreacted nitrous oxide from the phenol production step acid can be recovered in a second, or vent PSA step, and combined with the recovery of byproduct nitrous oxide waste streams from the production of adipic for the overall recovery of nitrous oxide, thereby significantly reducing nitrous oxide emissions from the production of adipic acid.
U.S. Pat. No. 6,348,083 B1 teaches an installation and a process for the recovery and/or purification of the nitrous oxide contained in a waste gas. The waste gas contains at least one portion of the nitrous oxide (N2O) and at least one other gaseous compound. The process further teaches that: (a) at least one nitrous oxide portion contained in the flow of waste gas is separated by permeation, and (b) at least a portion of the gaseous nitrous oxide separated in step (a) is recovered. Preferably, the source of the flow of waste gas is an industrial unit using an industrial process giving off the waste gas, preferably a unit for the production of adipic acid, a unit for the production of nitrous oxide, a unit for the production of glycoxylic acid or a unit for the production of nitric acid.
A disadvantage associated with the systems described above and other related art systems is that they are not capable of removing carbon dioxide impurity. It is well known to those skilled in the purification art that carbon dioxide is one of the most difficult impurities to be removed from a gas containing nitrous oxide.
Additionally, although useful in removing nitrogen, oxygen, hydrogen, carbon monoxide, and water impurities, the systems described above and other related art systems are not capable of removing other impurities such as ammonia, and methane. Carbon monoxide, ammonia, methane and hydrogen are light impurities.
U.S. Pat. No. 6,370,911 B1 provided a system and method for nitrous oxide purification, wherein the nitrous oxide product can be used in semiconductor manufacturing. The system and process involve a first sub-system having a purification tank for holding a liquefied nitrous oxide; a vaporizer in communication with the purification tank to receive, vaporize and convey a nitrous oxide vapor back to the purification tank; a distillation column disposed on a distal end of the purification tank to receive a nitrous oxide vapor; a condenser disposed on the distillation column, wherein light impurities are removed and a nitrous oxide devoid of light impurities is conveyed and converted into vapor in said vaporizer. A second sub-system having a first dry bed vessel is disposed downstream of the vaporizer to receive the vapor and reacting the acid gas therein; a second dry bed vessel downstream of the said first dry bed vessel for removing water and ammonia in the vapor. A third sub-system having a product tank wherein the purified nitrous oxide vapor is re-condensed; a transferring manifold including a liquid pump, liquid filter and a bypass for distributing the purified nitrous oxide to holding vessels. The invention allows for the removal of impurities and particulates, such that an ultra pure nitrous oxide product having a purity of 99.9998% or higher can be delivered to a point of use.
In this invention, a new N2O production scheme will be provided to purify the off gas stream or a waste gas flow containing nitrous oxide, by combination of wet scrubber absorption process, adsorption process, flash drum, and/or cryogenic distillation processes with reflux. The invention produces different levels of grade of nitrous oxide for different useful purposes.